This invention relates to a drainage pump, and more particularly a vertical shaft pump of all-level all-speed operation type which is operated at a constant speed irrespective of the suction water level.
The conventional vertical shaft pump used for rain water drainage is sometimes operated even when the water level at the suction side, namely the suction water level, is lower than a specified level in preparation for flooding. If the suction water level is low, air suction vortexes easily occur on the water surface on the suction side; as a result, the impulse of vortex produces vibration and noise, making the operation of the pump impossible. Therefore, various measures are taken to prevent vortexes on such a type of pump. FIG. 8 is a vertical cross sectional view of a conventional vertical shaft type axial flow pump. In this figure, when the suction water level W.L of intake chamber a is higher than the water level L.W.L corresponding to the limit suction water level at which a pump sucks water safely (critical submergence), a vertical shaft pump operates steadily at a speed of 100% because suction vortexes b are not produced. If the suction water level W.L is lower than the water level L.W.L, air suction vortexes easily occur on the water surface. When a pump is operated in the condition of the water level being lower than the water level L.W.L, the discharge rate of flow is limited by the reduction in rotational speed, or a splitter c is installed.
FIG. 9 is a front elevational view of another conventional vertical shaft type axial flow pump. On the vertical shaft pump shown in this figure, a suction tube d is longer than usual, an impeller e is mounted above the water level L.W.L corresponding to the limit suction water level, and a draining pipe f is installed immediately under the impeller e and connected to an air-water switching means g at the upper end. When the suction water level is above the water level L.W.L, the pump is operated steadily at a speed of 100%. If the suction water level becomes lower than the water level L.W.L, air is fed to a position immediately under the impeller e from the air-water switching means to perform the air-water separation because air suction vortexes b easily occur. Then, the pump is operated still at a speed of 100% under the condition of zero discharge, while the air and water are stirred.
As described above, when the suction water level W.L is below the water level L.W.L corresponding to the limit suction water level, speed reduction or air-water separation must be performed or a splitter must be installed for the conventional vertical shaft pump. For this reason, the conventional vertical shaft pump requires a control system for speed reduction, or requires a draining pipe f and an air-water switching means q. Although raising the position of the impeller e reduces vortexes, the water level at which pumping is started also rises, and the effective suction water depth of the vertical shaft pump decreases. Even if a splitter c is installed, the limit water depth h at which the pump does not suck air from the water surface on the suction side is about 1.2 times as large as the bore of the suction port d. To reduce this water depth h, it is necessary to restrict the pump discharge by controlling the rotational speed of the vertical shaft pump or the degree of opening of the discharge valve.
FIG. 10 is a view for explaining the construction of a conventional dry pit type pump. In this figure, this pump is installed outside an intake chamber a. The pump sucks water in the intake chamber through a suction tube i passing through the side wall of intake chamber a, pressurizes the water with an impeller j, and discharges it through the scroll chamber k. H.W.L denotes the highest level of water flowing into the intake chamber a, and L.W.L denotes the lowest water level at which the pump can operate safely without the occurrence of air suction vortexes or submerged vortexes.
On the above-described conventional dry pit type pump as well, air suction vortexes or submerged vortexes occur when the suction water level is lower than the lowest water level L.W.L, and vibration and noise may result because the pump sucks these vortexes. In such a case, it is necessary to decrease the discharge rate of flow or reduce the rotational speed.